First of all Welcome to the best site on the internet for what you are doing. Remember this, everyone here just wants to help. You may get differing opinions and you may not like the advice you get but it is never given to be mean or nasty, it is not tolerated here.
I spent about $4000 on each of my first two trips. I stayed about 12-14 days. My third trip I stayed for 3 months so you cant count that and this trip I am staying for a month. David 50 is right. Exchange your money in China but MAKE SURE IT IS CLEAN CRISP MONEY, the Banks will not take anything that is crinkly or torn or worn. It was a pain for me the last trip to change money in Beijing. I really do not know why that was such a hassle most times it is pretty simple. So before you leave go to your bank and change your money to new money. Now I dont even change anything. I just put the money in my wifes account and she handles it. Like Arnold said it makes life easier. I just keep a little for myself and she gets the rest. Now I dont recommend it the first time you are going to meet her. hehe But, afterwards if you are engaged and trust her, I would do it.
My wife is in Beijing and it can get expensive for an apartment or hotel compared to other cities. I have paid $100 per night and this last time I was paying $35 a night. But that is out of town a ways. Going to the Great Wall is not that expensive. I have done it twice. The first time was on a tour and cost me $50, but that was three places and included lunch. The last time we hired a car for the day. That ran about $73 plus lunch for him. Then we paid for our admissions. Taxi's are cheap and plentiful in Beijing. But just be aware that the driving here is nothing like you have ever seen in your life. No place and I mean No place on Earth is like here. Even after 3 months I was still jumpy every time we got in a cab. I have driven in about every major city in the us plus some around different countries, even Australia and nothing compares. Even Jamaica is not like here. So be prepared. Now be warned that if you do not know the driver, always take a taxi. They are easily identified and regulated. You pay the meter period. Private cars can be up to three times what you pay for a taxi. however going for the whole day can be different and that can be negotiated. When we do that I usually tip the driver depending on how he does. We have used the same guy all the time, he is fair and will be there when you want him. Most days he just sits in the car waiting for us. He pays the tolls and for gas. Make sure that is negotiated also. Usually let your lady do the bargaining. The minute most of them see a foreigner, the see dollar signs. They cannot get it out of their head that we are all rich and compared to most of them we are well off.
Communicating with your lady. if she has great English skills no problem. But, if she says her English is fair, i suggest a translator. the first days I used a guy i know there. The ones from the agency are usually overpriced. After that we bought a Besta with the pen so you can actually write on the screen. They run anywhere from $150 upwards. You can find them on E bay even. I guarantee you will use it a lot. Also carry a pocket dictionary with you in case the batteries run out. we still use ours from time to time when trying to get a point across.
As time gets nearer we can help you even more but these are some things to think about. Plus read read and read the threads here especially in the newbie area and ask an experienced member area. you will get plenty of great info there. I am not an original member here but I came on early in the facebook days and am a married member. i got the lady of my dreams here and I wish you the same. One last thing for now, different areas of the country have different customs, my experiences have been in Beijing, Wuhan and Xi'an. Others have had a little bit different experiences.
Best of luck to you.
Jim & Gina